1. Field of the Invention
The present invention relates to imager cells and, more particularly, to a bandgap tuned vertical color imager cell.
2. Description of the Related Art
A standard imager cell is a device that utilizes a photodiode to convert light energy into photo-generated electrons, and a number of transistors to control the operation of the photodiode and sense the number of photo-generated electrons that were produced by the light energy.
A color imager cell is a cell that produces photo information for more than one color, typically red, green, and blue. Digital cameras utilize millions of color imager cells that capture images based on the amount of reflected red, green, and blue light energy that strikes each imager cell.
One approach to implementing a color imager cell is to utilize three standard imager cells, and vary the depth of the sense point. Blue photons are high-energy photons that are absorbed at the surface of the photodiode, while green photons have less energy and are absorbed at a first depth that lies below the surface of the photodiode. Red photons have even less energy and are absorbed at a second depth that lies below the first depth.
Thus, by sensing the number of photo-generated electrons at the surface of a first cell, at the first depth in a second cell, and at the second depth in a third cell, blue, green, and red photo information, respectively, can be obtained from the three standared imager cells. One drawback of this approach, however, is that a color imager cell is three times the size of a standard imager cell.
Another approach to implementing a color imager cell that addresses the size problems of the three-cell approach is a vertical color imager cell. A vertical color imager cell is an imager cell that collects photo-generated electrons that represent different colors, such as red, green, and blue, at different depths in the same photodiode.
In an imager cell, the transistors required to operate the photodiode and sense the number of photo-generated electrons utilize a relatively small amount of space with respect to the amount of space utilized by the photodiode. Thus, one of the advantages of a vertical color imager cell is that since the cell collects multiple colors with the same photodiode, a vertical color imager cell is only slightly larger than a standard imager cell.
U.S. patent application Publication U.S. 2002/0058353 A1 published on May 16, 2002 describes a vertical color imager cell. FIG. 1 shows a combined cross-sectional and schematic diagram that illustrates a prior art color imager cell 100. Cell 100 is substantially the same as the cell shown in FIG. 2A of the '353 published application.
As shown in FIG. 1, imager cell 100 Includes a first p− region 110, a first n+ region 112 that contacts p− region 110, and a first depletion region 114 that is formed at the junction between regions 110 and 112. Imager cell 100 also includes a second p− region 120 that contacts n+ region 112, a second n+ region 122 that contacts p− region 120, and a second depletion region 124 that is formed at the junction between regions 120 and 122. In addition, imager cell 100 further includes a third p-type region 130 that contacts n+ region 122, a third n+ region 132 that contacts p− region 130, and a third depletion region 134 that is formed at the junction between regions 130 and 132.
In operation, as shown in FIG. 1, p− regions 110, 120, and 130 are connected to ground. In addition, n+ regions 112, 122, and 132 are connected to first, second, and third reset transistors 150, 152, and 154, respectively. Prior to collecting photo information, reset transistors 150, 152, and 154 are pulsed on which, in turn, places a positive potential on n+ regions 112, 122, and 132.
The positive potential reverse biases the pn junction of regions 110 and 112, thereby forming a red collecting photodiode, and the pn junction of regions 120 and 122, thereby forming a green collecting photodiode. The positive potential also reverse biases the pn junction of regions 130 and 132, thereby forming a blue collecting photodiode.
Once the positive potentials have been placed on n+ regions 112, 122, and 132, light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs.
The red electrons from the electron-hole pairs that are formed in depletion region 114 move under the influence of the electric field towards n+ region 112, where each additional electron collected by n+ region 112 reduces the positive potential that was placed on n+ region 112 by reset transistor 150. On the other hand, the holes formed in depletion region 114 move under the influence of the electric field towards p− region 110.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 110 within a diffusion length of depletion region 114, diffuse to depletion region 114 and are swept to n+ region 112 under the influence of the electric field. Further, the electrons that are formed in n+ region 112 remain in n+ region 112.
Similarly, the green electrons from the electron-hole pairs that are formed in depletion region 124 move under the influence of the electric field towards n+ region 122, where each additional electron collected by n+ region 122 reduces the positive potential that was placed on n+ region 122 by reset transistor 152. On the other hand, the holes formed in depletion region 124 move under the influence of the electric field towards p− region 120.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 120 within a diffusion length of depletion region 124, diffuse to depletion region 124 and are swept to n+ region 122 under the influence of the electric field. Further, the electrons that are formed in n+ region 122 remain in n+ region 122.
As with the red and green electrons, the blue electrons from the electron-hole pairs that are formed in depletion region 134 move under the influence of the electric field towards n+ region 132, where each additional electron collected by n+ region 132 reduces the positive potential that was placed on n+ region 132 by reset transistor 154. On the other hand, the holes formed in depletion region 134 move under the influence of the electric field towards p− region 130.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region 130 within a diffusion length of depletion region 134, diffuse to depletion region 134 and are swept to n+ region 132 under the influence of the electric field. Further, the electrons that are formed in n+ region 132 remain in n+ region 132.
After the red, green, and blue photodiodes have collected light energy for a period of time, known as the integration period, sense circuitry associated with the photodiodes detects the change in potential on n+ regions 112, 122, and 132. Specifically, in addition to a reset transistor, each photodiode also has an associated source follower transistor SF and a row select transistor RS.
The change in potentials on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the potential. Thus, when the gate of the row select transistor RS is pulsed, an output potential equal to the photodiode potential less a diode drop is output to a sense cell to determine the output potential. Once the change in positive potential has been determined, the photodiodes are reset and the process is repeated.
One problem with imager cell 100 is that the red, green, and blue photodiodes of cell 100 do not produce an equal number of photo-generated electrons when exposed to a white light source. The blue photodiode produces the largest number, with the green photodiode next and the red photodiode producing the smallest number of photo-generated electrons. The difference in the numbers of electrons must then be compensated for to produce an equal response.